The invention relates to a vacuum supply system for a brake power assist unit of a vehicle, as well as for at least one additional consuming device and has a vacuum generating device connected to a main pipe being provided with a first check valve between a suction connection of the vacuum generating device and the brake power assist unit. A secondary pipe leads at least indirectly from the vacuum generating device to the additional consuming device and this secondary pipe contains at least one flow control device for throttling the suction flow from the secondary consuming device to the vacuum generating device, as well as a valve for controlling the suction flow.
A vacuum supply system of this type is known from German Unexamined Patent Application DE-OS 31 24 153.
Check valves between vacuum generating devices and different consuming devices, or groups of consuming devices, are normally installed in the corresponding supply pipes in order to avoid incoming atmospheric air, due to leakages at individual consuming devices, which has a disadvantageous effect on the vacuum level at other consuming devices which are not leaking.
Flow control devices such as brake power assist units which are arranged in front of secondary or so-called vacuum-consuming comfort devices (e.g., headlight adjusting motors, air-conditioning system control motors, etc.) require vacuum supply priority, which is desirable for reasons of driving safety. With a secure priority system, such priority devices can be evacuated by the same vacuum generating device as the secondarY vacuum consuming devices.
It is known to use vacuum pumps with two separate suction connections (German Unexamined Patent Application DE-OS 32 43 984), in which case the suction connection belonging to the secondary consuming devices is equipped with a flow control device.
The above-mentioned arrangement is in use in diesel engine equipped vehicles manufactured by Daimler-Benz Aktiengesellschaft, in which however, a check valve is arranged in the suction pipe leading from the brake power assist unit to the vacuum source in order to maintain the vacuum level at the brake power assist unit during an atmospheric pressure flow short circuit of the vacuum pump. Between the brake power assist unit and the check valve, a throttled secondary pipe branches off from the suction pipe leading to the brake power assist unit. This throttled secondary pipe is bled only for the zero adjustment of the injection pump control rod when the diesel engine is shut off. For safety reasons, it must be a single pipe.
The growing number of comfort enhancing vacuum consuming devices used in motor vehicles make it necessary to be able to continuously ensure their vacuum supply. This supply should be improved over known systems mainly in view of the requirement that the lowest vacuum level be available at the brake power assist unit only when the power brake is actually actuated.
It is therefore the object of the invention to develop a vacuum supply system of this type, such that when the power brake is not actuated, the vacuum supply of secondary consuming devices is improved.
According to the invention, this object is achieved by having a valve that can be switched between at least two positions (I, II) as a function of the vacuum instantaneously existing in the system. The suction flow between the vacuum generating source and the consuming device(s) in position (I) of the valve is by an additional flow control device which valve is fluidly connected in series with the first flow control device, and wherein the valve in its other position (II) limits throttling only by the first control device.
By a bridging provided by the bypass, the check valve flow control device unit connected between the vacuum generating device and the secondary consuming devices, ensures that overall flow resistance of the supply of the secondary consuming devices is reduced, despite the additional flow control devices, so that when the power brake is not actuated, practically the complete vacuum output is available for use by the secondary consuming devices.
A vacuum supply system for a brake power assist unit and at least one additional consuming device is known from German Unexamined Patent Application DE-OS 31 05 041. There, the branch pipe of the secondary consuming device contains a flow control device for securing the priority of the supply of the brake power assist unit as well as a check valve, which fluidly is switched in parallel with this flow control device. Although the throttled pipe section is called a "detour pipe", it actuallY, because of the provided orientation of the parallel check valve through which an evacuation of the secondary consuming device must not be possible, represents the only bleeding possibility of the secondary consuming device. The check valve permits only an additional evacuation of the brake power assist unit at the expense of the vacuum level at the secondary consuming device.
Because of the specified opposite objectives, the mentioned German Unexamined Patent Application DE-OS 31 05 041 can supply no reference with respect to the construction of the type of vacuum supply system having a valve that can be switched between at least two positions (I, II) as a function of the vacuum instantaneously existing in the system. The suction flow between the vacuum generating source and the consuming device(s) in position (I) of the valve is by an additional flow control device which valve is fluidly connected in series with the first flow control device, and wherein the valve in its other position (II) limits throttling only by the first control device.
It is advantageous if the valve can be directly pneumatically switched within a certain vacuum value range by the vacuum existing in the system. The valve can be switched electromagnetically and in a controlled manner by an electric pressure switched responding to the vacuum existing in the system within a certain vacuum value range. The valve is constructed as a 2/2-way valve.
It is also advantageous if the valve is located in a chamber which has a suction connection path, a consuming device connecting path and an atmosphere connecting path and if both the suction connection path and the consuming device connecting path from the atmosphere connecting path are constantly separated by a diaphragm. The diaphragm can be switched within a certain range of the relationship between the vacuum at the suction connection path and the atmospheric pressure at the atmosphere connecting path and is connected with a valve body by which the switching of the valve takes place between the two positions (I and II). A throttling restriction is provided in the atmosphere connecting path of the valve.
Additionally, the additional flow control device of the valve is constructed as a narrowing of a round cross-section with a clear diameter of between 0.5 and 1.0 mm, preferably 0.8 mm, while the first flow control device has a clear diameter of between 0.8 and 1.2 mm, preferably 1.0 mm. In such a design, the additional flow control device can be integrated into the valve body. The valve should have an adjustable switching threshold in the vacuum valve range set between 0.7 and 0.4 bar, preferably at 0.5 bar. The valve should be arranged between the first flow control device and the secondary consuming device.
Special constructions of check valves combined with flow control devices are known in the vacuum supply systems of the above-mentioned types. However, in these, a flow control device becomes operative only when the check valve is opened against its restoring force. German Unexamined Patent Application DE-OS 28 14 384 describes a check valve having a flow control device which is integrated and fluidly is arranged in parallel.
By providing the check valve connected between the vacuum generating device and the consuming device(s) with a relatively strong return spring, it is ensured that the secondary consuming devices and the relatively wide flow control device, which is connected behind it (in parallel to the narrow bypass flow control device) will not be evacuated by means of this check valve before a required vacuum level is reached at the brake power assist unit. The priority of the supply of the brake power assist unit continues to be ensured, because as soon as its vacuum chamber is bled during braking operations, the (absolute) pressure in the suction pipe will rise, and the second check valve will close. A slight increase of the achievable (absolute) vacuum level as a result of the bypass pipe, while the consuming device output remains the same, can be accepted without any difficulties.
Another advantageous development for even more reliably ensuring the priority of the brake power assist unit from the vacuum supply system is achieved, if the second check valve, during each braking operation, can be power actuated by any of electrical, hydraulic or pneumatic means.
A check valve in a vacuum supply system for a brake power assist unit which can be power operated is known from German Patent DE-AS 11 20 293. In addition to being operated by pressure differences, it is also controlled by an electric switch coupled with the brake pedal. It is opened electromagnetically and connects the brake power assist unit with a vacuum compensating tank, in order to improve the vacuum level acting upon it.
According to another embodiment of the invention (shown in FIG. 4), a valve is arranged in a secondary pipe between the vacuum generating device and the at least one secondary consuming device and can be controlled as a function of the instantaneously existing vacuum and can be switched into at least two positions.
The secondary pipe either starts out directly from the vacuum generating device, or from its own suction connection (dual-flow system); or leads only indirectly to the secondary consuming device (single-flow system); i.e., leading away as a branch pipe, in front of the check valve, from the main pipe leading to the brake power assist unit.
The overall throttling of the secondary consuming device suction flow is varied, in that in one position of the valve an additional flow control device can be connected in series with the already existing flow control device. In the other valve position, no additional throttling takes place. The valve switches over with a given vacuum value range, i.e., at approximately 0.5 bar. Above this range (in the direction of the atmospheric pressure), the suction flow is throttled more, in order to continue to evacuate the brake power assist unit as a priority. Below this range, the vacuum level existing in the system is sufficient for a reliable operation of the brake power assist unit, so that the throttling of the secondary consuming device suction flow may be reduced without any loss of reliability, and thus the supply of the secondary consuming devices may be improved.
With respect to their basic principles, valve arrangements which allow a variable throttling of flows are known. German Unexamined Patent Application DE-OS 30 11 360 shows a compressed air system for a vehicle, which has a 3/2-way valve, which pneumatically can be switched into two positions and to two corresponding pipe sections. In one position, this 3/2-way valve switches a freely passable pipe section into a compressed air pipe and in the other position, switches a pipe section equipped with a control device into a compressed air pipe. By this arrangement, the amount of air is reduced which must be released by a pressure regulator connected behind the valve since the system pressure behind the valve, or in the pressure regulator, switches it, starting from a certain sufficient pressure level into a "throttling" position. In addition, from German Patent DE-PS 35 26 763, a similar arrangement of an electromagnetically switchable 3/2-way valve is known which, as a function of an electrically detected system pressure, switches either a throttled or an unthrottled pipe section between a bi-pressure pump and consuming devices. This arrangement has the purpose of making it possible to connect several consuming devices with different pressure level requirements to a single pressure source. However, neither of the latter texts supplies any reference to the endeavored improvement referenced in this invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.